Blasting initiator



April 21, 1959 w. E. SCHULZ BLASTING INITIATOR Filed Jan. 28, 1957 INVENTOR WILLIAM EARLE SCHULZ ATTORNEY nited BLASTIN G INITIATOR Application January 28, 1957, Serial No. 636,533

1 Claim. (Cl. 102-28) The present invention relates to an electric initiator device having novel design features. More particularly, this invention relates to an electric initiator device having enhanced resistance to accidental initiation because of static electricity discharges.

In the handling and use of electric initiator devices, such as squibs and detonators, an ever present danger is the presence of extraneous electrical energy, particularly in the form of accumulated static electricity, which may cause accidental and premature actuation of the device. In most of the electrically-actuated ignition devices, a resistance bridge wire is used in conjunction with an ignition charge sensitive to initiation by heat. By means of insulation and shunting devices, including the joining of the external ends of the electrical lead wires, passage of unwanted electrical current through the wires themselves can effectively be avoided, and very few premature initiations are due to such currents. However, the prevention of a discharge from the leg wires terminating in the initiator to the metal shell of the initiator is more difficult, particularly when the leg wires are exposed to electrical contact with a source of high voltage static electricity. Experience has shown that a human body can accumulate static electricity having a potential as high as 10,000 volts under conditions frequently encountered in the field. Since the human body has a capacitance which averages to about 0.0003 microfarad, the energy which can be delivered in the form of a static discharge is substantial, i.e., about 0.015 joule.

In the explosive industry, ignitable materials are classified as being static-sensitive or static-insensitive on the basis of whether or not they will become initiated by the discharge of static electricity having an energy of 0.015 joule or 10,000 man-equivalent volts, i.e., the maximum energy a man of average capacitance might accumulate. Many ignition compounds and compositions are known which are considered static-insensitive by this definition, and they are widely used whenever both fast action and initiation by relatively low energy are not required. However, no ignition compounds or composi tions are known which are both static-insensitive and susceptible to initiation rapidly by low energy. Therefore,

when a fast-acting initiation device which can be actuated by a low energy source is required, provision must be made to reduce the danger of premature initiation by static electricity which can be accumulated by personnel handling the electric initiators.

Obviously, no composition ignitable by heat will be insensitive to discharges of static electricity of all magnitudes; therefore, throughout this description and in the claim, the terms static-sensitive and static-insensitive are used in accordance with the foregoing definition.

A number of expedients for overcoming static susceptibility of electric initiators are known, and most have been effective to some degree. However, none have been entirely satisfactory when cost, ease of assembly, number of manipulative steps, and efiectiveness are all considered.

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Accordingly, an object of this invention is to provide an initiator device with improved resistance to initiation by discharges of static electricity. A further object is to provide an initiation device having such properties which is simple to fabricate and does not require special costly ingredients. Other objects will become apparent as this invention is more fully described.

I have found that the foregoing objects may be achieved when I provide an initiation device which is essentially identical with the conventional devices having an ignition composition surrounding a bridge wire except that the bridge wire and ignition composition are not located at the terminal ends of the leg wires, but instead are positioned such that a portion of each terminal end projects beyond the ignition composition, and the terminal ends are surrounded by a static-insensitive composition ignitable by the flash from the bead. Preferably, the static-resistant composition is one containing metal particles so that it is voltage-sensitive, that is, has a high resistance at low voltage, and a relatively low resistance at high voltage.

To more fully describe my invention, reference is now made to the accompanying drawing which is illustrative only, the invention not being limited thereto.

Referring now to the drawing, 1 represents a metal shell, 2 is a sealing plug, for example of rubber-like material, held in shell 1 by the two peripheral crimps, 3 are leg wires having terminal ends 3a within shell 1, 4 is insulation about the leg wires 3, 5 is a resistance wire, for example Nichrome, spanning the leg wires 3, 6 is an ignition charge, for example, lead styphnate in a polyvinyl acetate binder, and 7 is a static insensitive charge ignitable by the ignition charge, for example a 30/35/35 mixture of magnesium/barium peroxide/selenium (by weight).

It will be understood that the size of the initiator dcpicted in the drawing is greatly exaggerated, inasmuch as most initiators actually used have internal diameters of only 0.25 inch and may vary in length from about 0.50 inch to several inches, depending on the amount of the charge 7 required and on the presence of other charges. Charge 7 need not be the main charge but may act as an initiating charge for the main charge. Also, charge 6 does not have to be in the form of a bead, but may be a layer of loose charge or may be a paste in a cavity of the plug. Essentially, the construction of the initiator differs from the construction of conventional electric initiators only in the requirement that the leg wires extend beyond the ignition composition and are surrounded by a static insensitive charge.

I believe the enhanced static resistance obtained with the present design is due to the well-known fact that a static discharge will usually emanate from a point or sharp edge rather than from a rounded continuous surface, and that the terminal ends of the leg wires are separated from the static-sensitive ignition composition a distance sufiicient so that the intervening static-insensitive material prevents the discharge spark, if any, from igniting the static-sensitive composition. My experiments have shown that static resistance is not appreciably enhanced if the terminal ends are not surrounded by a static-insensitive charge.

As previously mentioned, I prefer to use a metal-containing static-insensitive charge which is sensitive to voltage. At low voltages, for example 6 to 440, such composition has sufficient resistance that only insignificant amounts of current will be shunted through the mixture instead of passing through the bridge wire when a firing current is applied. 0n the other hand, when the voltages encountered with static electricity, several thousand volts and up, are impressed between the shell wall and the leg wire, the composition is sufficiently conductive to drain off most of the energy .of the static charge.

In order to illustrate my invention further, reference is now made to the following examples.

Example 1 A number of initiators wereprepared which were identicalin allrespects except for the length of the leg wires extending below the rubber plug, i.e., the bridge posts. In all cases, an aluminum shell inch in length, 0.233 inch in outside diameter, and having a 0.010-inch-wall thickness was charged with approximately 2 grains of a 50/25/ 25 mixture of smokeless powder/lead salt of dinitro orthocresol/potassium chlorate. The rubber plug assembly,.including the leg wires, the bridge wire, .and the ignition bead, was then pressed intotheshell byhanduntil further entry was resisted by the compacting .ofthe main charge. The plug was crimped in- .place. Ineach case, the bridge wire was ia0.00l-inch-:diameter. platinum iridium alloy connected across the bridge posts :at 21-point about ,5 inch below the bottom edge of. the rubber plug, the bridge posts beingspaced'apart approximately inch. An ignition bead .of approximately 1 2 milligrams of lead styphnate in a polyvinyl acetate binder cornplctely surrounds the bridge wire. The surfaceofthe bead, at the posts extended about inch on each side of the junction between the bridge wire and the bridge posts.

In case A, the posts were about V inch in.length,i.e., the ends were covered by the head. .In case B the posts were about 42 inch in length, i.e., about inch of post extended beyond the surface of .the bead. In case C, the bridge posts were 4 inch in length, i.e., about' inch of post extended beyond .the surface of the bead.

The initiators were tested by determining the minimum discharge of static electricity between the leg wires and shell wall required to initiate the initiator, of each being tested. In the following table are shown the lowest, the highest, and the average amount of energy (injoules) required to initiate the initiators.

Lowest Highest Average Energy Energy Energy oules) oules) (joules) When initiatorsidentical with those :described for Case "B were charged with a 30/35/35 mixture of powdered magnesium/ barium peroxide/selenium instead of the nonconductive mixture previously described, 'the test results were as .follows:

Lowest Highest Average Energy Energy Energy oules) oules) (joules) Case D 0. 040 0.200 0.055

0.015 joule). 'When the ends of the posts are in a nonconductive material, the sameprojcction is borderline, and somewhat greater projection is desirable. In the production of initiators in mass quantities, precision with of an inch is readily obtainable. Therefore, the minimum amount of projection of the ends of the bridge posts beyond the surface of the ignitioncomposition should be at least V inch. Obviously, the maximum amount of projection is limited only by shell size and production problcms. The amount of projection required is not a ratio of any dimension of the assembly, but is fixed by the requirement that the path of the energy flow from the end of the bridge post to the shell be far enough from the ignition composition so that no ignition of the composition can occur. Thus, the minimum amount of projection is constant for different size initiators, leg wire spacings, etc.

The invention has been illustrated by examples in which the ignition composition contained lead styphnate. Obviously, the effect of the projection of the bridge posts is applicable to the other static-sensitive ignition compounds or compositions used in initiators. In the Bureau of Mines Report-of Investigation No. 5002, issued September 1953, and entitled Sensitivity of Explosives to Initiation by Electrostatic Discharges, the energy required for Zero-probability ignition of representative materials is listed in Table 3. The following materials which are classified as static sensitive because they are susceptible to ignition by 0.015 joules or less are included in this list.

Material tested: Energy (joules) Lead azide 0.007 Lead styphnate 0.0009 DDNP (diazodinitrophenol) 0.012 Tetracene 0.010 Potassium chlorate/lead sulfocyanate mixture 0.00075 Other ignition compositions, .such as lead nitrato hypophosphite, cuprous acetylide, and lead picrate, are similarly classifiable as static sensitive materials.

The :number of static insensitive materials is far greater,'and.includes mostexplosive compounds and compositionnot included in the list of static sensitive materials. .Since this invention is specific to the extension of the bridge posts outside of theignition composition, many variations may be made without departure from the scope of this invention. I intend, therefore, to be limited only by the following claim.

I claim:

An initiation .device .having reduced susceptibility. to static comprising an elongated tubular metal shell having one integrally closed end, a sealing plug closing the opposite end of said shell, a .pair ofspaced leg wires passing throughsaid sealing plug and terminating within said shell, a resistance .bridge wire connecting said leg wires at points intermediate the inner end of said sealing plug and the terminal ends of said leg'wires, an ignition composition surrounding said bridge wire, the terminal ends of said legwires extending beyond said ignition composition, and a static-insensitive charge ignitable by said ignition composition surrounding said terminal ends of said leg wires, said static-insensitive charge comprising a metal-containing mixture which is voltageasensitive.

References Cited in the file of this patent UNITED STATES PATENTS 2,103,014 Palmieri et al. Dec. 21, 1937 2,408,125 Rolfes Sept. 24, 1946 2,410,801 .Audrieth Nov. 12, 1946 2,428,218 Hale et a1. Aug. .16, 1949 2,481,696 Seavey Sept. 13, 1949 

